If proliferation activity of tumor cells can be determined non-invasively by image diagnosis, it will be help for evaluation of growth rate and malignancy of the tumor. Detection of the most rapidly growing regions of a tumor by image diagnosis will be useful in preparing plans for radiation fields in radiotherapy and identifying suitable portions for biopsy. Such methods will permit an early and accurate evaluation of therapeutic effects, which is difficult to identify by CT- or MRI-based anatomical evaluation or PET-based measurement of glucose-metabolic changes. Particularly, they will be useful for an early assessment of therapeutic effects of anticancer agents that may cause strong side effects.
In order to solve these clinically important problems, use of 5-iodo-deoxyuridine labeled with a radioactive iodine and thymidine labeled with carbon-11 which is a positron-emitter, have been studied (Tjuvajev J G et al., J. Nucl. Med. 35, pp.1407–1417 (1994); Blasberg R G et al, Cancer Res. 60, pp.624–635 (2000); Martiat Ph et al., J. Nucl. Med. 29, pp.1633–1637 (1998); Eary J F et al., Cancer Res. 59, pp. 615–621 (1999); U.S. Pat. Nos. 5,094,835; 5,308,605). It is considered that these radiolabeled compounds are taken into cells as precursors for DNA synthesis required for cell division of rapidly-growing tumors, and then phosphorylated by thymidine kinase, followed by incorporation into DNA, to reflect proliferation activity of the tumor. These radiolabeled compounds, however, are decomposed rapidly in vivo, making it difficult to perform non-invasive evaluation of the proliferation activity of the tumor. The method using carbon-11-labeled thymidine, in particular, requires very complicated mathematical model analysis, and cannot become popular as a diagnostic technique of nuclear medicine imaging.
The rapid metabolic decomposition of these radiolabeled compounds in vivo is considered to be due to cleavage of C—N glycosidic bonds by thymidine phosphorylase and instability of the labels in vivo. If the C—N glycosidic bonds are cleaved, the compound loses its affinity to tumors, thereby decreasing in accumulation of radioactivity in tumors, while the radioactive metabolites increase background radioactivity, thereby making imaging of the tumors difficult.
To solve these problems, radiolabeled compounds with metabolic stability have been synthesized by introducing fluorine atoms, which are high in electronegativity, to the 2′ or 3′ position in certain nucleosides, and have been studied for imaging of tumors. Thus, 3′-deoxy-3′-fluorothymidine that contains fluorine 18, a positron emitter, at the 3′ position shows a high stability in vivo and an accumulation in tumor tissue (Shields A F et al., Nature Med. 4, pp.1334–1336 (1998)). Though this radiolabeled compound is stable in vivo, it is a radio-labeled compound with a short-life positron emitter, and therefore a cyclotron is required in the hospital, limiting the usage of the compound. For this radiolabeled compound, the major process responsible for its accumulation in cells is the phosphorylation caused by thymidine kinase that is an index of DNA synthesis, and thus it does not serve as an agent that essentially reflects DNA synthesis.
A derivative of 5-iododeoxyuridine, in which fluorine is introduced to the 3′ position in the same manner as above to increase its stability in vivo, has recently been reported. Though stable in vivo, however, this radiolabeled compound was high in retention in blood and failed to show a significant accumulation in a tumor compared to 5-iododeoxyuridine (Choi S R et al., J. Nucl. Med. 41, p. 233 (2000)).
2′-fluoro-5-iodoarabinouridine, in which fluorine is introduced to the 2′ position, shows a high stability in vivo, and has been used for identification of introduction and expression in vivo of a vector for gene therapy, utilizing a phosphorylation reaction specific to thymidine kinase of human herpesvirus. It has also been applied to image diagnosis for virus infection, based on the high specificity to the viral thymidine kinase (Tjuvajev J G et al., Cancer Res. 56, pp.4087–95 (1996); Tjuvajev J G et al., Cancer Res. 58, pp.4333–4441 (1998); Wiebe L I et al., Nucleosides Nucleotides 18, 1065–1076 (1999); Gambhir S S et al., Nucl. Med. Biol. 26, pp.481–490 (1999); Haubner R et al., Eur. J. Nucl. Med. 27, pp.283–291 (2000); Tjuvajev J G et al. Cancer Res. 59, 5186–193 (1999); Bengel F M et al., Circulation 102, pp.948–950 (2000)).
In view of the above situation, the present invention aims to provide a radiolabeled compounds that are practically useful in clinical fields, stable in vivo, and able to retain in cells after being phosphorylated by thymidine kinase of mammals, or reflect the DNA synthesis activity after being incorporated in DNA, particularly those compounds which are labeled with a single-photon emitter to achieve a wide spectrum of use, and also aims to provide methods for diagnosis of tissue proliferation activity and for treatment of proliferative disease, utilizing agents that contain said radiolabeled compounds.